The surface acoustic wave device includes the surface acoustic wave element having comb electrodes formed on the surface of a piezoelectric substrate, and has an electric characteristic fit for a resonant circuit or a filter by a surface acoustic wave propagating between the comb electrodes.
Therefore, when enclosing the surface acoustic wave element in the surface acoustic wave device, space is necessary at least above the surface of the piezoelectric substrate on which the comb electrodes of the surface acoustic wave element are formed.
Further, if dust, moisture or the like is attached to the comb electrodes, propagation characteristic of the surface acoustic wave is varied. To prevent this, it is desired that the room above the surface on which the comb electrode is formed is airtight sealed.
One method to satisfy the above requirement, a technique has been disclosed in the official gazette of the Japanese Unexamined Patent Publication No. 2000-124767 (hereafter referred to as prior art.) In this prior art, a bump is employed for connecting the substrate with the surface acoustic wave element (chip), and sealing walls are formed on both inside and outside the bump.
On examination of this configuration, an area is required to form the inside wall between the comb electrode and the bump, which becomes disadvantageous to the miniaturization of the surface acoustic wave element.
Further, the surface of the surface acoustic wave element facing the opposite side to the substrate is bare, which lacks reliability. To solve this problem, in the embodiment of the prior art, the surface acoustic wave element is sealed inside the bottom of a sealing case, using the aforementioned double wall structure. Further, this sealing case is mounted on the substrate in the state that the surface acoustic wave element faces upward.
However, with the above configuration, the surface acoustic wave device becomes inevitably large in size because of the sealing case.
Considering this point, the inventor of the present invention has proposed a structure of the surface acoustic wave device, which includes a surface acoustic wave element, having a substrate and a comb electrode formed on a piezoelectric substrate, flip-mounted on the substrate with a bump in a state such that the comb electrode faces the substrate; a first resin layer formed in the surrounding area of the bumps of the surface acoustic wave element; and a second resin layer covering the first resin layer and at least the side faces of the surface acoustic wave element (the Japanese Patent Application No. 2000-29880.)
In regard to this surface acoustic wave device, a cross-section structure and an exemplary manufacturing process are shown in FIGS. 1 and 2, respectively. As shown in FIG. 1, a surface acoustic wave element 10 (shown as C) having a comb electrode 11 formed on a piezoelectric substrate, and a substrate 20 (shown as B) having electrode patterns on both faces via a through hole 21 are prepared. Next, according to the process shown in FIG. 2, surface acoustic wave element 10 is chip bonded onto substrate 20 with a pad electrode 12, so that the face on which comb electrode 11 is formed is disposed opposite to substrate 20 (process P1).
Next, resin material forming a first resin layer (a) coats a pad electrode 12, an electrode pattern 22 and the side faces of surface acoustic wave element 10 using a dispenser, etc (process P2). Here, as the resin material forming the first resin layer (a), a liquid resin having a high viscosity is employed so as not to flow inside pad electrode 12.
Next, drying process follows for 15–30 minutes at 125–150° C. (process P3). After the drying, resin material of the second resin layer (b) having higher viscosity than the resin material of the first resin layer (a) is transfer molded, and one face of substrate 1 including surface acoustic wave element 10 is sealed, followed by heat curing the resin (process P4). The heating conditions at this time are, for example, for 3–5 minutes at 150–175° C.
Further, as a post cure, heat is added for 60–180 minutes at 150–175° C. (process P5).
Here, although not shown in FIG. 1, it is possible to form a plurality of surface acoustic wave devices at a time by chip bonding a plurality of surface acoustic wave elements 10 on a single substrate in the above process P1 and thereafter performing the above processes P2 to P4. In this case, the plurality of surf ace acoustic wave devices are cut into separated pieces by dicing (process P6). Thereafter, a characteristic test is performed for each surface acoustic wave device (process P7), and pieces having satisfactory quality is selected, packed and shipped (process P8).
According to the above-mentioned method having been proposed by the inventor of the present invention, a small-sized surface acoustic wave-device having a thin profile can be attained by adopting the structure of surf ace acoustic wave element 10 bonded on substrate 20. However, resin sealing by heating must be performed twice, which may cause a problem of the thermal stress received by the piezoelectric substrate forming surface acoustic wave element 10 becomes twofold.